1. Field of the Invention
The present invention relates to an optical encoder that detects a positional displacement.
2. Description of the Related Art
Conventionally, an optical encoder is known as a displacement detection device that detects displacement of an object to be measured, such as an amount of movement and an amount of rotation. The optical encoder includes a light source, a scale which reflects or transmits light emitted from the light source and which can be displaced with respect to the light source, and a photo detector that receives the light reflected from the scale or the light transmitted through the scale. A pattern which reflects or transmits light is formed on the scale. The amount of light received by the photo detector is changed by the relative displacement of the scale. The displacement is detected on the basis of a detection signal generated by the change of the amount of light in the photo detector.
In EP2093543, such an optical encoder is disclosed. Specifically, as shown in FIG. 9, a method is disclosed in which a large amount of information can be obtained from a small number of scale tracks by forming a plurality of modulations having different pitches in a scale pattern on one track.
In addition to the prior example described above, according to the study of the inventors of the present invention, it has been known that there are unnecessary spatial frequency components described below in an image received on a sensor surface via the scale including a plurality of modulation pitches. Specifically, the unnecessary spatial frequency components are unnecessary spatial frequency components generated by interference between two diffracted light beams and unnecessary spatial frequency components included in transmittance distribution or reflectivity distribution of the scale. By the effect of the above, an error from an ideal sine wave occurs. As a result, a problem becomes obvious in which a position detection error occurs when converting the sine wave into a position signal by an arctangent calculation.
To perform position detection with a high degree of accuracy, it is necessary to reduce an error component of a detected wave by reducing the unnecessary spatial frequency components generated by interference between two diffracted light beams and the unnecessary spatial frequency components included in transmittance distribution or reflectivity distribution of the scale in the image received on the sensor surface. FIG. 2 shows an entire schematic configuration of an optical encoder considered to be an example of a method for solving this problem. Specifically, this example is a method for switching a detection resolution and obtaining a signal by using a scale as shown in FIG. 3 and a light receiving surface arrangement and a sensor signal processing circuit shown in FIGS. 6 to 8. This method will be described below as a comparative example of the present invention.